Tuesday, February 9, 2016

LPC4078 operational

I am continuing my project of doing robot stuff with a buddget of zero, plus the stuff on my bench. Well, the stuff on my bench includes a Loginator2368 purple board, and a box of LPC4078 Cortex-M4 microcontrollers. Those controllers happen to be pin-compatible with the LPC2368 the board was designed for. In this case that means that all the power pins have the same jobs and voltage levels (even if they have different names and perhaps different internal connections), all the special pins like reset are in the same place, and all the GPIO pins have the same numbers, and where the 2368 and 4078 have the same peripherals, they have compatible pin assignments.

One thing that the 4078 has that the 2148 doesn't is internal pullup resistors. I can take advantage of these to reduce the part count in several places.

Therefore I got out my soldering iron and finally attached the board and LPC4078. I was trying to figure out what was the minimum amount of components I could get away with, since I can't find my solder paste stencil for this board, and would have to manually solder everything. I decided to skip the power section, so no external battery or regulator. I skipped the USB section, the LEDs, and the voltage reference. I also skipped the crystal, since the part has an internal RC oscillator which is good enough for now. I even skipped all of the bypass caps.

The only part that it looked like I absolutely needed was the reset button, since my first read on the datasheet didn't show the reset pin as having a pullup. It turns out that it does, so, I didn't even need that. I did end up stealing a push button from another board and using it as a reset switch.

I did the normal SMD IC soldering thing. I carefully lined up the part on the board, then used a normal soldering iron and normal solder to glob all the pins on each side down, and to each other. I then used wick to clean up all the bridges. This naturally leaves the connections to the board intact. I then soldered on the reset button, more as a convenience than anything. (Next time, leave a reset terminal on the edge!) Also solder a shield 6-pin top-and-bottom connector onto the power and serial connections.

Then, hook it up to power. No smoke, the chip isn't hot, it passes the smoke test. But, I've seen this board not work before, with the LPC2368 that it was designed for. So, the real test is running the ISP.


It works!!! I had to guess at the frequency it wants, since I don't have an oscillator attached at all. The internal RC oscillator runs at 12MHz, so I put that in.

Now to massively restructure the code. A Cortex-M4 is quite a different beast than an ARM7TDMI. It has an FPU and built-in VIC instead of the VIC as a peripheral. It also doesn't use 32-bit ARM instructions, but (mostly) 16-bit Thumb2 instructions. The 4078 is also a much different part than the 2148, with a different set of registers in different places. But, probably 90% of the code can be in common, if we carefully separate the code into the part which is different and the part which is common. That's the next big adventure. Throw one switch in the Makefile and compile for a 4078 instead of 2148.

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